The double dose of flooding and earthquakes that hit the Bay of Plenty in July were not connected and their timing was just plain bad luck, a scientist says.

Dr Tony Hurst of the Institute of Geological and Nuclear Sciences said the earthquakes were not a "swarm" as first thought, but a single big quake and a series of aftershocks.

The big tremor on July 18 measured 5.4 on the Richter scale, making it the biggest earthquake in the whole Taupo volcanic zone, stretching from Mt Ruapehu to White Island, since the Edgecumbe quake of 1987.

Although it was centred at Lake Rotoehu on the rim of the huge ancient Okataina volcano, Dr Hurst told the NZ Geological Society's annual conference in Taupo this week that it was not triggered by volcanic activity either.

The main fault line associated with the volcano runs east-west at Rotoehu, but the quake shook the land along a separate northeast-southwest fault.

Dr Hurst said the tremors could have been triggered by heavy rain if the level of the Rotorua lakes had risen enough to increase the pressure on the hot rocks underneath, which are unusually near the surface in the Taupo volcanic zone.

"But the lakes didn't rise rapidly enough, only a few centimetres," he said.

"It seems to have been purely bad luck that the earthquake coincided with the storms."

Dr Hurst said the July quake, like most in the volcanic zone, was caused by the east coast being gradually stretched apart from the rest of the North Island at the leading edge of the great Australian "plate".

"The land is pulling apart. At Edgecumbe it actually created a gap."